1. Field of the Invention
The present invention relates to an apparatus for detecting an over current of a motor for use in a vehicle. More particularly, this invention is directed to an over current detection apparatus of a motor in a vehicle which is capable of preventing the motor from burning out by more precisely detecting an over current of the motor within an overall drive area thereof, wherein this is achieved by setting, as an over current detection reference current, a predefined range between a normal current and a constraint current of the motor by drive input conditions of the motor, detecting a current of the motor flowing in a field effect transistor (FET) via a shunt resistor and a differential amplifier, and comparing the detected current with the over current detection reference current.
2. Description of Related Art
In general, a motor is known to provide a driving force necessary for vehicle operation, wherein a power required by the vehicle is supplied by adjusting rotary power of the motor under the control of a microprocessor. Such a motor may burn out due to over current that occurs due to internal, external, or constraint effects, etc., during the operation of the motor.
As such, since the prior art motor burns out due to over current unless such over current is accurately detected and the operation of the motor is stopped to prevent burn out upon the detection of the over current, an over current detection of the motor has conventionally been performed through an over current detection device as illustrated in FIG. 1, in order to prevent the above phenomenon.
FIG. 1 is a circuit diagram showing a configuration of an over current detection device of a motor used in a vehicle according to the prior art. This prior art device comprises an input unit 1 for inputting an external signal including motor drive information with duty cycle, a microprocessor 2 that provides an FET drive control signal to drive an FET according to the motor drive information in the external signal from the input unit 1, determines whether or not there occurs an over current state in the motor based on an output from a comparison unit 9 to be described below, and controls the motor to halt its operation upon the occurrence of the over current, an FET driver 3 for generating an FET drive signal in response to the FET drive control signal provided by the microprocessor 2, an FET 4 as a motor driving element that is turned ON/OFF depending on the FET drive signal from the FET driver 3, a freewheeling diode 5 for passing a recovery current for continuous operation of the motor when the FET 4 is OFF, the motors 6 that is driven according to the operations of the FET 4 and the freewheeling diode 5 and provides a driving force necessary for the vehicle's operation, a voltage detector 7 for detecting a voltage that is taken between both ends of the FET 4 produced by a current flowing in an internal resistor of the FET 4, a reference voltage setting unit 8 for setting an over current detection voltage for the motor 6 as a reference voltage, and the comparison unit 9 for comparing the voltage detected by the voltage detector 7 with the reference voltage set by the reference voltage setting unit 8 and providing the microprocessor 2 with an output voltage of high level implying the normal state if the detected voltage is less than the reference voltage and with an output voltage of low level meaning the issuance of the over current state if the detected voltage is above the reference voltage.
FIGS. 2A and 2B show output waveforms resulting when the motor is in the normal state and over current state, respectively, according to the conventional over current detection device. With reference to these drawings, an operation of the conventional over current detection device will be described in detail below.
At first, the microprocessor 2 included in the prior art over current detection device inputs the motor drive information in the external signal from the input unit 1 and controls the FET driver 3 according to the motor drive information so as to drive the FET 4. Upon the ON operation of the FET 4, the motor 6 is driven by a power (or supply voltage); and upon the OFF operation of the FET 4, the recovery current issuing from the motor 6 enables it to be continuously driven by passing the current through the freewheeling diode 5.
The FET 4 includes an internal resistor and the ON operation of the FET 4 is controlled according to the motor drive information. Based on the current variation by such ON operation of the FET 4, the voltage between both ends of the internal resistor in the FET 4 is varied.
After that, a comparator contained in the comparison unit 9 compares the voltage between both ends of the FET 4 detected by the voltage detector 7 with the reference voltage provided by the reference voltage setting unit 8. From the comparison, the comparator provides the microprocessor 2 with an output voltage of 5 V if the detected voltage is less than the reference voltage, and with an output voltage of 0 V if not, i.e., if the detected voltage is above the reference voltage.
Then, the microprocessor 2 determines that the motor 6 is in the normal state if the output voltage from the comparison unit 9 is at a high level of 5 V; and, conversely, it determines that an over current state is present in the motor 6 if the output voltage is at a low level of 0 V, and halts the operation of the motor in order to prevent the motor from burning out due to the occurrence of the over current.
In the conventional over current detection device of the motor for use in the vehicle as mentioned above, however, there exist some problems as set forth below.
That is, since the internal resistor in the FET exhibits slight variation due to manufacturing tolerances and thus is varied depending upon the temperature for the same FET, a dispersion of the detected voltage by the current flowing in the FET becomes large.
Consequently, there may be a problem that the power necessary for vehicle operation is not supplied thereto due to the interruption of the motor that arises when the detected voltage exceeds the reference voltage upon their comparison, even under the normal state. In particular, in the case of an engine cooling motor, the engine may overheat, causing the vehicle to break down.
Further, under the condition that an over current may occur, if the motor is driven continuously as the detected voltage is less than the reference voltage to thereby make an issuance of a larger current, there ultimately exists an instance where the motor burns out.
In addition, since the reference voltage set by the reference voltage setting unit for the over current detection cannot be modified, it may be determined that the detected voltage is less than the reference voltage, upon the issuance of the over current of the motor when the motor is driven with low current. Accordingly, over current of the motor is not detected, leading to motor burn out.